1. Field of the Invention
The invention relates to a new system for immobilizing electrolyte containing silica in lead acid batteries. This system relies on the use of silica particles from sources that have not been used in prior art electrolyte immobilizing systems and it also relies on the ability of separator materials and of the porous electrodes of the lead acid battery to filter silica particles from an electrolyte consisting of a mixture of sulfuric acid and silica particles. As a consequence of this filtering action there is a relative increase in the concentration of silica particles in the residual unfiltered electrolyte to levels at which gel formation occurs. In addition, gel formation can be caused to occur as a consequence of the increase in sulfate ion concentration in the electrolyte which occurs as the lead acid battery is being charged, either initially, after formation of positive and negative plates therein, or after use. Charging causes a rise in sulfate ion concentration in the electrolyte, which is accompanied by a rise of the specific gravity of the sulfuric acid electrolyte. This new method of immobilizing electrolyte is particularly suited for “VRLA” or valve regulated lead acid batteries which are sealed, especially those in which the plates are formed in the battery case.
2. Description of the Prior Art
Lead acid batteries of two distinct types are made; one type is known as a flooded battery and the other as a VRLA battery. In the former, the electrolyte is liquid sulfuric acid having a density of 1.220 to 1.320. When either of these types of battery is being charged, electrolytic decomposition of water releases oxygen gas at the positive electrodes and hydrogen gas at the negative electrodes. Hydrogen and oxygen that are released escape from a flooded battery, so that there is a loss of water from the electrolyte, necessitating its replacement. The electrolyte is immobilized in the VRLA battery, and a one-way valve prevents external gases from entering the battery. The valve will allow gas to escape from the interior of the battery when a certain internal pressure is exceeded. The immobilization of the electrolyte in a valve regulated lead acid battery makes it possible for the gases that are generated at one electrode to have access to the other electrode; as a consequence, oxygen gas travels inside the battery and is reduced at the surface of the negative plates and is returned to the electrolyte of the battery. This process, which is called the “Internal Oxygen Cycle”, is the foundation of the operation of VRLA batteries. Unfortunately, even when the internal oxygen cycle is well established, some of the water is lost because the hydrogen gas generated at the negative electrodes cannot be oxidized at the positive electrodes and thus any hydrogen evolution means a corresponding loss of water.
In general, lead acid batteries have assemblies of alternating positive and negative plates with separator material between adjacent plates. The assemblies can be produced by assembling lead grids pasted with positive active material, lead grids pasted with negative active material, and separator material, making the necessary electrical connections, inserting the assembly into a case, and charging an electrolyte. The pasted lead grids must be converted to positive and negative battery plates. This is accomplished by what is called “formation”, which involves passing an electric current through the assembly while it is in a suitable electrolyte, and can be carried out either in a tank before the assembly is inserted into its case, or in the case.
At least three methods are known for immobilizing the electrolyte in a VRLA battery, and page 80 of a publication which predates VRLA batteries, and is entitled Electric Accumulators, refers to a “jelly form of electrolyte due to Dr. Schoop [which] was made by adding dilute sulphuric acid to a solution of silicate of sodium” as one of two illustrations of the statement that “[a]ttention was at one time given to the production of solid accumulators . . . .” A first one of the methods for immobilizing the electrolyte in a VRLA battery involves the reaction of sulfuric acid with silica particles to form a silica gel. Examples of this involving the use of fumed silica can be found in U.S. Pat. No. 4,414,302, “Jache, et al.” and other examples involving the use of colloidal silica can be found in U.S. Pat. No. 4,317,872, “Varma”. According to another method, liquid electrolyte is immobilized when it is absorbed and retained in a very absorbent glass fiber mat separator. See, for example, U.S. Pat. No. 4,465,748, “Harris”. In a third method, which is disclosed in U.S. Pat. No. 5,128,218, “Tokunaga et al.”, fine silica particles are used in place of glass fiber separator.
Jache et al. discloses discharging a battery or a plate, either immediately after formation or after a charging of the final battery, so that sulfate anions from the sulfuric acid of the electrolyte are bonded to the electrodes as lead sulfate, removing at least a part of the electrolyte which remains in the battery, adding sulfuric acid and a gelling agent or sulfuric acid, a gelling agent and phosphoric acid to the electrolyte removed, and recharging the removed electrolyte plus the added sulfuric acid, gelling agent, and phosphoric acid, if any, to the battery. The amounts of the gelling agent, which can be silicic acid, and of sulfuric acid added to the removed electrolyte are sufficiently small that the electrolyte, when returned to the battery, does not gel until the battery is recharged, Upon recharging, however, the sulfate anions which had been bonded to the electrodes as lead sulfate are returned to the electrolyte, and raise the sulfate anion concentration enough that gelling of the electrolyte occurs. After slight drying, cracks form in the gel through which oxygen gas can be transported in the vapor phase for recombination.
Harris discloses a highly absorptive glass fiber separator for immobilizing the electrolyte in a VRLA battery wherein from 5 to 35% by weight of the fibers have a diameter of less than 1 μm.
Tokunaga et al. discloses a sealed tubular lead acid battery in which an absorbent separator is formed from fine primary particles of hydrous silicon dioxide. The Example of the patent describes the production of an assembly of positive plates, negative plates and “separators provided with projections”, and insertion of the assembly into a container. The “separators provided with projections” are really spacers; they are positioned between the positive plates and the negative plates, and perform the function of positioning the plates relative to one another. They can be composed, the Example says, of a foamed phenolic material.
After the foregoing assembly is inserted into a container, the reference says, “a powder 8 (FIGS. 1 and 2) is packed both between the plates and around” the assembly. The “powder comprises fine primary particles of hydrous silicon dioxide diameters of 10-40 millimicrons and specific surface areas of 100-150 m2/g. The primary particles agglomerate to form secondary particles with diameters of 50-200 microns. This powder is highly flowable and has an angle of repose of 25-30 degrees. Having such high fluidity, the powder can be closely packed into the container within a short time by applying vibrations with an amplitude of 1-2 mm under an acceleration of gravity of 2-4 g.”
As noted above with reference to Jache et al., the immobilizing technique that involves the use of silica particles relies on the fact that, upon slight drying, the silicate gels that are formed crack and the oxygen gas is transported through the cracks for recombination.
Silica and sulfuric acid are the raw materials from which silica gels in batteries are produced. The following discussion reviews the characteristics of various types of silica that are used in lead acid batteries, as well as the current techniques used to produce silica gels. Silica is used both as the main ingredient in microporous polymer/silica separators and also as the gelling agent in immobilizing the electrolyte in an important segment of VRLA batteries.
In the microporous separator application, the silica used is dried precipitated silica. In the electrolyte gelling application, either colloidal silica or fumed silica, is added to cause sulfuric acid to gel. These silicas have properties which differ as a consequence of the processes used to make them. Precipitated silica is made by reacting sodium silicate (water glass) with sulfuric acid. As the reaction between the two ingredients proceeds, primary particles of the silicate are formed as distinct polymer chains that vary in length, depending on the precipitation reaction conditions.
Primary particles of silica have a great affinity for each other, they coalesce quickly into aggregates, which are not stable either, forming agglomerates of silica when they come together. These agglomerates are the silica particles that get separated from the reaction solution when precipitated silica is dried in either a kiln or in a spray drier.
The average agglomerate has a cross section of 10 to 20 microns, while aggregates are one order of magnitude smaller than agglomerates and primary particles of silica are from one to two orders of magnitude smaller than the aggregates.
Colloidal silica is a suspension of extremely fine amorphous silica particles in water. Colloids do not settle out of suspension over time. The highest concentration of commercially available colloidal silica is 50%. Silica colloids in colloidal silica have particle sizes generally in the range of 5 to 50 nanometers. Fumed silica is an amorphous form of silica formed by the combustion of silicon tetrachloride in hydrogen-oxygen furnaces. It is therefore very light and very fine. Primary particles of fumed silica typically have particle sizes of a few nanometers. Precipitated silica, as used herein, refers to amorphous silica that is precipitated from a solution.